OBJECTIVE To determine the peptide binding motif of a rhesus macaque MHC class I molecule. RESULTS We have now defined the optimal SIV gag CTL epitope restricted by the rhesus MHC class I molecule Mamu-A*01 and defined a general peptide binding motif for this molecule that is characterized by a dominant position 3 anchor (proline). We used peptide elution and sequencing, peptide binding assays, and bulk and clonal CTL assays to demonstrate that the optimal Mamu-A*01-restricted SIV gag CTL epitope was CTPYDINQM181P189. Mamu-A*01 is unique in that it is found at a high frequency in rhesus macaques, and all SIV-infected Mamu-A*01-positive rhesus macaques studied to date develop an immunodominant gag-specific CTL response restricted by this molecule. DISCUSSION The majority of immunogenic CTL epitopes bind to MHC class I molecules with high affinity. However, peptides longer or shorter than the optimal epitope rarely bind with high affinity. Therefore, identification of optimal CTL epitopes from pathogens may ultimately be critical for inducing strong CTL responses and developing epitope-based vaccines. The SIV-infected rhesus macaque is an excellent animal model for HIV infection of humans. Although a number of CTL epitopes have been mapped in SIV-infected rhesus macaques, the optimal epitopes have not been well defined, and their anchor residues are unknown. FUTURE DIRECTIONS Identification of the optimal SIV gag CTL epitope will be critical for a variety of studies designed to induce CD8+ CTL responses specific for SIV in the rhesus macaque. KEY WORDS MHC, SIV, CTL, motif, vaccine FUNDING NIH RR00167, AI32426, AI41913 PUBLICATIONS Allen, TM, Sidney, J, del Guercio, M-F, Lehman, E, Glickman, RL, Lensmeyer, GL, Wiebe, DA, Pauza, CD, Johnson, RP, Sette, A, and D.I. Watkins. 1998. Characterization of the Peptide Binding Motif of a Rhesus MHC class I molecule (Mamu-A*01) that Binds an Immunodominant CTL Epitope from SIV. J Immunol. 160:6062-6071.